Crevasse on the Gorner Glacier, Zermatt, Switzerland.

A crevasse is a crack or fissure in a glacier or snow field. Falling into a hidden crevasse is a mountaineer's worst nightmare. Falling into a visible crevasse is usually the result of an error in judgement. Anyone planning on walking on glaciers should be trained in crevasse rescue.

As a glacier flows down a mountain, its pace quickens when the slope steepens. When ice flows faster on steeper pitches of ice below slower moving ice above, a crescentic crevasse may form.

Glaciers move slower down the sides than in the centre because of the friction between the ice and walls of the mountain. This speed difference often creates lateral crevasses near the sides of the glacier, pointing up-valley.

Reference

Mountaineering: The Freedom of the Hills, 5th edition. ISBN 0-89886-309-0